The class is crazy powerful. Those 5 levels of archmage can nearly triple the effectiveness of a character compared to if they had just stayed as sorc/wizard.
Mastery of Elements, Mastery of Shaping, and Spell Power +6 can turn a wizard into an unstoppable juggernaut of destruction. I ran a party of 18th level characters through the Bastion of Broken Souls. Everyone had a tough time, except the wiz13/archmage5. When his initiative came up, practically everything they were fighting took 48d6 of acid damage (2xMet Swarm - I don't allow sonic, and they were fighting fiends). None of the party did, of course, despite the fact that the fight was raging around them (mastery of shaping).
On top of that, the DC for the saves was a 41 (32 int, SF/GSF, SP+6), and he was getting +28 to punch through SR (SP/GSP/SP+6). Everything took horrifying amounts of damage, except the party.
The thing about the archmage is that it not only is capable of super-powering spell DCs and SR penetration, but Mastery of Shaping and Mastery of Elements remove all tactical restrictions from a boomer mage. Once a boomer has the two of those, there's nothing really hampering them from cutting loose with giant area effects every round.
Sure, it costs spell slots. Once you hit high enough level, the bonus spells you get from your casting stat counteract this, and the benefits you get WAY outweigh any possible cost.